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  • 1. Amendola, Luca
    et al.
    Appleby, Stephen
    Avgoustidis, Anastasios
    Bacon, David
    Baker, Tessa
    Baldi, Marco
    Bartolo, Nicola
    Blanchard, Alain
    Bonvin, Camille
    Borgani, Stefano
    Branchini, Enzo
    Burrage, Clare
    Camera, Stefano
    Carbone, Carmelita
    Casarini, Luciano
    Cropper, Mark
    de Rham, Claudia
    Dietrich, Joerg P.
    Di Porto, Cinzia
    Durrer, Ruth
    Ealet, Anne
    Ferreira, Pedro G.
    Finelli, Fabio
    Garcia-Bellido, Juan
    Giannantonio, Tommaso
    Guzzo, Luigi
    Heavens, Alan
    Heisenberg, Lavinia
    Heymans, Catherine
    Hoekstra, Henk
    Hollenstein, Lukas
    Holmes, Rory
    Hwang, Zhiqi
    Jahnke, Knud
    Kitching, Thomas D.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kunz, Martin
    La Vacca, Giuseppe
    Linder, Eric
    March, Marisa
    Marra, Valerio
    Martins, Carlos
    Majerotto, Elisabetta
    Markovic, Dida
    Marsh, David
    Marulli, Federico
    Massey, Richard
    Mellier, Yannick
    Montanari, Francesco
    Mota, David F.
    Nunes, Nelson J.
    Percival, Will
    Pettorino, Valeria
    Porciani, Cristiano
    Quercellini, Claudia
    Read, Justin
    Rinaldi, Massimiliano
    Sapone, Domenico
    Sawicki, Ignacy
    Scaramella, Roberto
    Skordis, Constantinos
    Simpson, Fergus
    Taylor, Andy
    Thomas, Shaun
    Trotta, Roberto
    Verde, Licia
    Vernizzi, Filippo
    Vollmer, Adrian
    Wang, Yun
    Weller, Jochen
    Zlosnik, Tom
    Cosmology and fundamental physics with the Euclid satellite2018In: Living Reviews in Relativity, ISSN 1433-8351, E-ISSN 1433-8351, Vol. 21, p. 1-345, article id 2Article, review/survey (Refereed)
    Abstract [en]

    Euclid is a European Space Agency medium-class mission selected for launch in 2020 within the cosmic vision 2015-2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and red-shifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

  • 2. Ashoorioon, Amjad
    et al.
    Casadio, Roberto
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Anisotropic non-gaussianity from rotational symmetry breaking excited initial states2016In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 12, article id 002Article in journal (Refereed)
    Abstract [en]

    If the initial quantum state of the primordial perturbations broke rotational invariance, that would be seen as a statistical anisotropy in the angular correlations of the cosmic microwave background radiation (CMBR) temperature fluctuations. This can be described by a general parameterisation of the initial conditions that takes into account the possible direction-dependence of both the amplitude and the phase of particle creation during inflation. The leading effect in the CMBR two-point function is typically a quadrupole modulation, whose coefficient is analytically constrained here to be vertical bar B vertical bar less than or similar to 0.06. The CMBR three-point function then acquires enhanced non-gaussianity, especially for the local con figurations. In the large occupation number limit, a distinctive prediction is a modulation of the non-gaussianity around a mean value depending on the angle that short and long wavelength modes make with the preferred direction. The maximal variations with respect to the mean value occur for the con figurations which are coplanar with the preferred direction and the amplitude of the non-gaussianity increases (decreases) for the short wavelength modes aligned with (perpendicular to) the preferred direction. For a high scale model of inflation with maximally pumped up isotropic occupation and epsilon similar or equal to 0. 01 the difference between these two configurations is about 0. 27, which could be detectable in the future. For purely anisotropic particle creation, the non-Gaussianity can be larger and its anisotropic feature very sharp. The non-gaussianity can then reach f(NL)similar to 30 in the preferred direction while disappearing from the correlations in the orthogonal plane.

  • 3. Ashoorioon, Amjad
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Hemispherical anomaly from asymmetric initial states2016In: Physical Review D, ISSN 2470-0010, Vol. 94, no 4, article id 043009Article in journal (Refereed)
    Abstract [en]

    We investigate if the hemispherical asymmetry in the CMB is produced from asymmetric excited initial conditions. We show that in the limit where the deviations from the Bunch-Davies vacuum are large and the scale of new physics is maximally separated from the inflationary Hubble parameter, the primordial power spectrum is modulated only by position-dependent dipole and quadrupole terms. Requiring the dipole contribution in the power spectrum to account for the observed power asymmetry, A = 0.07 +/- 0.022, we show that the amount of quadrupole terms is roughly equal to A(2). The mean local bispectrum, which gets enhanced for the excited initial state, is within the 1 sigma bound of Planck 2015 results for a large field model, f(NL) similar or equal to 4.17, but is reachable by future CMB experiments. The amplitude of the local non-Gaussianity modulates around this mean value, depending on the angle that the correlated patches on the 2d CMB surface make with the preferred direction. The amount of variation is minimized for the configuration in which the short and long wavelength modes are around the preferred pole and vertical bar(k(3)) over right arrow vertical bar approximate to vertical bar(k) over right arrow (l approximate to 10)vertical bar << vertical bar(k(1)) over right arrow vertical bar approximate to vertical bar(k(2)) over right arrow vertical bar approximate to vertical bar(k) over right arrow (l approximate to 2500)vertical bar with f(NL)(min) approximate to 3.64. The maximum occurs when these modes are at the antipode of the preferred pole, f(NL)(max) approximate to 4.81. The difference of non-Gaussianity between these two configurations is as large as similar or equal to 1.17, which can be used to distinguish this scenario from other scenarios that try to explain the observed hemispherical asymmetry.

  • 4. Beltran Jimenez, Jose
    et al.
    Heisenberg, Lavinia
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Helsinki Institute of Physics, Finland; Helsinki University, Finland.
    Teleparallel Palatini theories2018In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 8, article id 039Article in journal (Refereed)
    Abstract [en]

    The Palatini formalism, which assumes the metric and the affine connection as independent variables, is developed for gravitational theories in flat geometries. We focus on two particularly interesting scenarios. First, we fix the connection to be metric compatible, as done in the usual teleparallel theories, but we follow a completely covariant approach by imposing the constraints with suitable Lagrange multipliers. For a general quadratic theory we show how torsion naturally propagates and we reproduce the Teleparallel Equivalent of General Relativity as a particular quadratic action that features an additional Lorentz symmetry. We then study the much less explored theories formulated in a geometry with neither curvature nor torsion, so that all the geometrical information is encoded in the non-metricity. We discuss how this geometrical framework leads to a purely inertial connection that can thus be completely removed by a coordinate gauge choice, the coincident gauge. From the quadratic theory we recover a simpler formulation of General Relativity in the form of the Einstein action, which enjoys an enhanced symmetry that reduces to a second linearised diffeomorphism at linear order. More general theories in both geometries can be formulated consistently by taking into account the inertial connection and the associated additional degrees of freedom. As immediate applications, the new cosmological equations and their Newtonian limit are considered, where the role of the lapse in the consistency of the equations is clarified, and the Schwarzschild black hole entropy is computed by evaluating the corresponding Euclidean action. We discuss how the boundary terms in the usual formulation of General Relativity are related to different choices of coordinates in its coincident version and show that in isotropic coordinates the Euclidean action is finite without the need to introduce boundary or normalisation terms. Finally, we discuss the double-copy structure of the gravity amplitudes and the bootstrapping of gravity within the framework of coincident General Relativity.

  • 5. Beltrán Jiménez, Jose
    et al.
    Heisenberg, Lavinia
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Coincident general relativity2018In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 98, no 4, article id 044048Article in journal (Refereed)
    Abstract [en]

    The metric-affine variational principle is applied to generate teleparallel and symmetric teleparallel theories of gravity. From the latter we discover an exceptional class which is consistent with a vanishing affine connection. Based on this remarkable property, this work proposes a simpler geometrical formulation of general relativity that is oblivious to the affine spacetime structure, thus fundamentally depriving gravity of any inertial character. The resulting theory is described by the Hilbert action purged from the boundary term and is more robustly underpinned by the spin-2 field theory, where an extra symmetry is now manifest, possibly related to the double-copy structure of the gravity amplitudes. This construction also provides a novel starting point for modified gravity theories, and the paper presents new and simple generalizations where analytical self-accelerating cosmological solutions arise naturally in the early-and late-time Universe.

  • 6. Beltrán Jiménez, Jose
    et al.
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Modified Gravity with Vector Distortion and Cosmological Applications2017In: Universe, ISSN 2218-1997, Vol. 3, no 2, article id 47Article in journal (Refereed)
    Abstract [en]

    We briefly review the basics of Weyl geometry and its natural extension by a general linear distortion of the metric connection by a vector field. A special class of the connections has torsion but retains the Weyl's semi-metricity condition. We present ghost-free gravitational theories in this geometrical setup and highlight their possible cosmological applications, such as new self-tuning solutions and new bouncing solutions found in the quadratic-curvature theories. The vector distortion can mimic the cosmological effects of dark matter.

  • 7. Capozziello, Salvatore
    et al.
    Harko, Tiberiu
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Lobo, Francisco S. N.
    Olmo, Gonzalo J.
    Hybrid Metric-Palatini Gravity2015In: Universe, ISSN 2218-1997, Vol. 1, no 2, p. 199-238Article, review/survey (Refereed)
    Abstract [en]

    Recently, the phenomenology of f(R) gravity has been scrutinized. This scrutiny has been motivated by the possibility to account for the self-accelerated cosmic expansion without invoking dark energy sources. Besides, this kind of modified gravity is capable of addressing the dynamics of several self-gravitating systems alternatively to the presence of dark matter. It has been established that both metric and Palatini versions of these theories have interesting features but also manifest severe and different downsides. A hybrid combination of theories, containing elements from both these two formalisms, turns out to be also very successful accounting for the observed phenomenology and is able to avoid some drawbacks of the original approaches. This article reviews the formulation of this hybrid metric-Palatini approach and its main achievements in passing the local tests and in applications to astrophysical and cosmological scenarios, where it provides a unified approach to the problems of dark energy and dark matter.

  • 8. Golovnev, Alexey
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). National Institute of Chemical Physics and Biophysics, Estonia.
    Cosmological perturbations in modified teleparallel gravity models2018In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 11, article id 012Article in journal (Refereed)
    Abstract [en]

    Cosmological perturbations are considered in f (T) and in scalar-torsion f (phi) T teleparallel models of gravity. Full sets of linear perturbation equations are accurately derived and analysed at the relevant limits. Interesting features of generalisations to other teleparallel models, spatially curved backgrounds, and rotated tetrads are pointed out.

  • 9. Golovnev, Alexey
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Oslo, Norway.
    Effectively nonlocal metric-affine gravity2016In: Physical Review D, ISSN 2470-0010, Vol. 93, no 6, article id 064081Article in journal (Refereed)
    Abstract [en]

    In metric-affine theories of gravity such as the C-theories, the spacetime connection is associated to a metric that is nontrivially related to the physical metric. In this article, such theories are rewritten in terms of a single metric, and it is shown that they can be recast as effectively nonlocal gravity. With some assumptions, known ghost-free theories with nonsingular and cosmologically interesting properties may be recovered. Relations between different formulations are analyzed at both perturbative and nonperturbative levels, taking carefully into account subtleties with boundary conditions in the presence of integral operators in the action, and equivalences between theories related by nonlocal redefinitions of the fields are verified at the level of equations of motion. This suggests a possible geometrical interpretation of nonlocal gravity as an emergent property of non-Riemannian spacetime structure.

  • 10. Golovnev, Alexey
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    On the covariance of teleparallel gravity theories2017In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 34, no 14, article id 145013Article in journal (Refereed)
    Abstract [en]

    The basics of teleparallel gravity and its extensions are reviewed with particular emphasis on the problem of the Lorentz-breaking choice of connection in pure-tetrad versions of the theories. Various possible ways to covariantise such models are discussed. A by-product is a new form of f (T) field equations.

  • 11. Jimenez, Jose Beltran
    et al.
    Heisenberg, Lavinia
    Koivisto, Tosmi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Cosmology for quadratic gravity in generalized Weyl geometry2016In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 4, article id 046Article in journal (Refereed)
    Abstract [en]

    A class of vector-tensor theories arises naturally in the framework of quadratic gravity in spacetimes with linear vector distortion. Requiring the absence of ghosts for the vector field imposes an interesting condition on the allowed connections with vector distortion: the resulting one-parameter family of connections generalises the usual Weyl geometry with polar torsion. The cosmology of this class of theories is studied, focusing on isotropic solutions wherein the vector field is dominated by the temporal component. De Sitter attractors are found and inhomogeneous perturbations around such backgrounds are analysed. In particular, further constraints on the models are imposed by excluding pathologies in the scalar, vector and tensor fluctuations. Various exact background solutions are presented, describing a constant and an evolving dark energy, a bounce and a self-tuning de Sitter phase. However, the latter two scenarios are not viable under a closer scrutiny.

  • 12.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Helsinki Institute of Physics, Finland; Helsinki University, Finland.
    Nyrhinen, Hannu J.
    Stability of disformally coupled accretion disks2017In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 92, no 10, article id 105301Article in journal (Refereed)
    Abstract [en]

    The no-hair theorem postulates that the only externally observable properties of a black hole are its mass, its electric charge, and its angular momentum. In scalar-tensor theories of gravity, a matter distribution around a black hole can lead to the so called 'spontaneous scalarisation' instability that triggers the development of scalar hair. In the Brans-Dicke type theories, this effect can be understood as a result of tachyonic effective mass of the scalar field. Here we consider the instability in the generalised class of scalar-theories that feature non-conformal, i.e. 'disformal', couplings to matter. Such theories have gained considerable interest in the recent years and have been studied in a wide variety of systems, both cosmological and astrophysical. In view of the prospects of gravitational wave astronomy, it is relevant to explore the implications of the theories in the strong-gravity regime. In this article, we concentrate on the spontaneous scalarisation of matter configurations around Schwarzschild and Kerr black holes. We find that in the more generic scalar-tensor theories, the instability of the Brans-Dicke theory can be enhanced, suggesting violations of the no-hair theorem. On the other hand, we find that, especially if the coupling is very strong, or if the gradients in the matter distribution are negligible, the disformal coupling tends to stabilise the system.

  • 13.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Saridakis, Emmanuel N.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Pontificia Universidad Catolica de Valparaiso, Chile.
    Tamanini, Nicola
    Scalar-fluid theories: cosmological perturbations and large-scale structure2015In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 9, article id 047Article in journal (Refereed)
    Abstract [en]

    Recently a new Lagrangian framework was introduced to describe interactions between scalar fields and relativistic perfect fluids. This allows two consistent generalizations of coupled quintessence models: non-vanishing pressures and a new type of derivative interaction. The implications of these to the formation of cosmological large-scale structure are uncovered here at the linear order. The full perturbation equations in the two cases are derived in a unified formalism and their Newtonian, quasi-static limit is studied analytically. Requiring the absence of an effective sound speed term in the coupled dark matter fluid restricts the Lagrangian to be a linear function of the matter number density. This leaves new potentially viable classes of both algebraically and derivatively interacting models wherein the coupling may impact the background expansion dynamics and imprint new signatures into the large-scale structure.

  • 14.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Tamanini, Nicola
    A note on viability of nonminimally coupled f(R) theory2016In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 48, no 7, article id 97Article in journal (Refereed)
    Abstract [en]

    Consistency conditions for nonminimally coupled f(R) theories have been derived by requiring the absence of tachyons and instabilities in the scalar fluctuations. This note confirms these results and clarifies a subtlety regarding different definitions of sound speeds.

  • 15. Nersisyan, Henrik
    et al.
    Akrami, Yashar
    Amendola, Luca
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Rubio, Javier
    Dynamical analysis of R1/square R-2 cosmology: Impact of initial conditions and constraints from supernovae2016In: Physical Review D, ISSN 2470-0010, Vol. 94, no 4, article id 043531Article in journal (Refereed)
    Abstract [en]

    We discuss the cosmological implications of the R square R-2 nonlocal modification to standard gravity. We relax the assumption of special initial conditions in the local formulation of the theory, perform a full phase-space analysis of the system, and show that the late-time cosmology of the model exhibits two distinct evolution paths, on which a large range of values for the present equation of state can be reached. We then compare the general solutions to supernovae data and place constraints on the parameters of the model. In particular, we find that the mass parameter of the theory should be smaller than 1.2 in Hubble units.

  • 16. Solomon, Adam R.
    et al.
    Enander, Jonas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Akrami, Yashar
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Könnig, Frank
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cosmological viability of massive gravity with generalized matter coupling2015In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 4, article id 027Article in journal (Refereed)
    Abstract [en]

    There is a no-go theorem forbidding flat and closed FLRW solutions in massive gravity on a flat reference metric, while open solutions are unstable. Recently it was shown that this no-go theorem can be overcome if at least some matter couples to a hybrid metric composed of both the dynamical and the fixed reference metric. We show that this is not compatible with the standard description of cosmological sources in terms of effective perfect fluids, and the predictions of the theory become sensitive either to the detailed field-theoretical modelling of the matter content or to the presence of additional dark degrees of freedom. This is a serious practical complication. Furthermore, we demonstrate that viable cosmological background evolution with a perfect fluid appears to require the presence of fields with highly contrived properties. This could be improved if the equivalence principle is broken by coupling only some of the fields to the composite metric, but viable self-accelerating solutions due only to the massive graviton are difficult to obtain. These problems can be avoided by giving the reference metric dynamics.

  • 17.
    Sundell, Peter
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Anisotropic cosmology and inflation from a tilted Bianchi IX model2015In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 92, no 12, article id 123529Article in journal (Refereed)
    Abstract [en]

    The dynamics of the tilted axisymmetric Bianchi IX cosmological models are explored allowing energy flux in the source fluid. The Einstein equations and the continuity equation are presented treating the equation of state w and the tilt angle of the fluid lambda as time-dependent functions, but when analyzing the phase space w and lambda are considered free parameters and the shear, the vorticity and the curvature of the spacetime span a three-dimensional phase space that contains seven fixed points. One of them is an attractor that inflates the universe anisotropically, thus providing a counterexample to the cosmic no-hair conjecture. Also, examples of realistic though fine-tuned cosmologies are presented wherein the rotation can become significant towards the present epoch but the shear stays within the observational bounds. The examples suggest that the model used here can explain the parity-violating anomalies of the cosmic microwave background. The result significantly differs from an earlier study, where a nonaxisymmetric Bianchi IX type model with a tilted perfect dust source was found to induce too much shear for observationally significant vorticity.

  • 18. van de Bruck, Carsten
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Longden, Chris
    Disformally coupled inflation2016In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 3, article id 006Article in journal (Refereed)
    Abstract [en]

    A disformal coupling between two scalar fields is considered in the context of cosmological inflation. The coupling introduces novel derivative interactions mixing the kinetic terms of the fields but without introducing superluminal or unstable propagation of the two scalar fluctuation modes. Though the typical effect of the disformal coupling is to inhibit one of the fields from inflating the universe, the energy density of the other field can drive viable near Sitter-inflation in the presence of nontrivial disformal dynamics, in particular when one assumes exponential instead of power-law form for the couplings. The linear perturbation equations are written for the two-field system, its canonical degrees of freedom are quantised, their spectra are derived and the inflationary predictions are reported for numerically solved exponential models. A generic prediction is low tensor-to-scalar ratio.

  • 19. van de Bruck, Carsten
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Longden, Chris
    Non-Gaussianity in multi-sound-speed disformally coupled inflation2017In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 2, p. 1-29, article id 029Article in journal (Refereed)
    Abstract [en]

    Most, if not all, scalar-tensor theories are equivalent to General Relativity with a disformally coupled matter sector. In extra-dimensional theories such a coupling can be understood as a result of induction of the metric on a brane that matter is confined to. This article presents a first look at the non-Gaussianities in disformally coupled inflation, a simple two-field model that features a novel kinetic interaction. Cases with both canonical and Dirac-Born-Infeld (DBI) kinetic terms are taken into account, the latter motivated by the possible extra-dimensional origin of the disformality. The computations are carried out for the equilateral con figuration in the slow-roll regime, wherein it is found that the non-Gaussianity is typically rather small and negative. This is despite the fact that the new kinetic interaction causes the perturbation modes to propagate with different sounds speeds, which may both significantly deviate from unity during inflation.

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